Motion track markers go red and stop tracking
The tracking process stops when blender cannot accurately track an element. In other words: tracking fails when the tracked element cannot be found accurately within the search box area.

This can be because the element has moved too much and is outside the search area, or because is blurred, deformed, occluded or too close to the edge of the frame. The tracking process also stops when it has reached the last frame defined for the scene (or the first one if tracking backwards). 

What are the blue and red lines?

When you track there is a path created showing the motion of the tracked element across the frame.

Red indicates motion on the previous frames, blue is for the path on future frames.

If you track forwards you will see a red track being created. If you track backwards you will create a blue path.

When you play back you can see both.



What can you do when the tracking stops?

Go back one frame (or where the tracking stopped) Change some settings and re-start the tracking process.

If the tracked object is growing in size or has moved beyond the search area make the search box larger.

(To view the search box enable the Search Marker Display or preass Alt+S) 



Change the motion model. Usually changing from Loc to LocRotScale or perspective works well.



For tracked features that change in shape because of the perspective or because they change size, you might want to change the match setting from keyframe (where the tracker uses the original shape as reference to find the matching features) to Previous Frame (where it will try to match the features to those in the previous frame only)